Renewable Energy Sources

LESSON 21. Basics of Solar Photovoltaic’s

Photovoltaic and Photovoltaic Cells

When sunlight strikes a PV cell, the photons of the absorbed sunlight dislodge the electrons from the atoms of the cell. The free electrons then move through the cell, creating and filling in holes in the cell. It is this movement of electrons and holes that generates electricity. The physical process in which a PV cell converts sunlight into electricity is known as the photovoltaic effect. One single PV cell produces up to 2 watts of power, too small even for powering pocket calculators or wristwatches. To increase power output, many PV cells are connected together to f form modules, which are further assembled into larger units called arrays. This modular nature of Fundamentals of Photovoltaic Materials

PV enables designers to build PV systems with various power output for different types of applications.A complete PV system consists not only of PV modules, but also the “balance of system” (BOS)- the support structures, wiring, storage, conversion devices, etc. i.e. everything else in a PVsystem except the PV modules.

Two major types of PV systems are available in the marketplacetoday: flat plate and concentrators.As the most prevalent type of PV systems, flat plate systems build the PV modules on a rigid andflat surface to capture sunlight. Concentrator systems use lenses to concentrate sunlight on the PVcells and increase the cell power output. Comparing the two systems, flat plate systems aretypically less complicated but employ a larger number of cells while the concentrator systems usesmaller areas of cells but require more sophisticated and expensive tracking systems. Unable tofocus diffuse sunlight, concentrator systems do not work under cloudy conditions.Types of PV cell materialsPV cells are made of semiconductor materials. The major types of materials are crystalline and thinfilms, which vary from each other in terms of light absorption efficiency, energy conversionefficiency, manufacturing technology and cost of production. The rest of the paper discusses thecharacteristics, advantages and limitations of these two major types of cell materials.

Converting Photons to Electrons

The solar cells that you see on calculators and satellites are photovoltaic cells or modules (modules are simply a group of cells electrically connected and packaged in one frame). Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity. Once used almost exclusively in space, photovoltaics are used more and more in less exotic ways. They could even power our houses.

Photovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. In fact, Over 95% of the solar cells produced worldwide are composed of the semiconductor material silicon (Si). Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric fields that act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

1. Crystalline Materials

1.1 Single-crystal silicon

Single-crystal silicon cells are the most common in the PV industry. The maintechnique for producing single-crystal silicon is the Czochralski (CZ) method.High-purity polycrystalline is melted in a quartz crucible. A single-crystal siliconseed is dipped into this molten mass of polycrystalline.

Fundamentals of Photovoltaic Materials

Slowly from the melt, a single-crystal ingot is formed. The ingots are then sawedinto thin wafers about 200-400 micrometers thick (1 micrometer = 1/1,000,000meter). The thin wafers are then polished, doped, coated, interconnected andassembled into modules and arrays.A single-crystal silicon has a uniform molecular structure. Compared to non-crystallinematerials, its high uniformity results in higher energy conversionefficiency is the ratio of electric power produced by the cell to the amount ofavailable sunlight power i.e. power-out divided by power-in. The higher a PVcell’s conversion efficiency, the more electricity it generates for a given area ofexposure to the sunlight. The conversion efficiency for single-silicon commercialmodules ranges between 15-20%. Not only are they energy efficient, single-siliconmodules are highly reliable for outdoor power applications.The average price for single-crystal modules is $3.97 per peak watt in 1996.(Renewable Energy Annual 1997). About half of the manufacturing cost comesfrom wafering, a time-consuming and costly batch process in which ingots are cutinto thin wafers with a thickness no less than 200 micrometers thick. If the wafersare too thin, the entire wafer will break in wafering and subsequent processing.Due to this thickness requirement, a PV cell requires a significant amount of rawsilicon and half of this expensive material is lost as sawdust in wafering.

1.2 Polycrystalline silicon

Consisting of small grains of single-crystal silicon, polycrystalline PV cells are lessenergy efficient than  single-crystalline silicon PV cells. The grain boundaries inpolycrystalline silicon hinder the flow of electrons and reduce the power output ofthe cell. The energy conversion efficiency for a commercial module made ofpolycrystalline silicon ranges between 10 to 14%.A common approach to produce polycrystalline silicon PV cells is to slice thinwafers from blocks of cast polycrystalline silicon. Another more advancedapproach is the “ribbon growth” method in which silicon is grown directly as thinribbons or sheets with the approach thickness for making PV cells. Since nosawing is needed, the manufacturing cost is lower. The most commerciallydeveloped ribbon growth approach is EFG (edge-defined film-fed growth).Compared to single-crystalline silicon, polycrystalline silicon material is strongerand can be cut into one-third the thickness of single-crystal material. It also hasslightly lower wafer cost and less strict growth requirements. However, theirlower manufacturing cost is offset by the lower cell efficiency. The average pricefor a polycrystalline module made from cast and ribbon is $3.92 per peak watt in19962, slightly lower than that of a single-crystal module.

1.3 Gallium Arsenide (GaAs)

A compound semiconductor made of two elements: gallium (Ga) and arsenic (As),GaAs has a crystal structure similar to that of silicon. An advantage of GaAs is thatit has high level of light absorptivity. To absorb the same amount of sunlight,GaAs requires only a layer of few micrometers thick while crystalline siliconrequires a wafer of about 200-300 micrometers thick.3 Also, GaAs has a muchhigher energy conversion efficiency than crystal silicon, reaching about 25 to 30%.Its high resistance to heat makes it an ideal choice for concentrator systems in whichcell temperatures are high. GaAs is also popular in space applications where strongresistance radiation damage and high cell efficiency are required.The biggest drawback of GaAs PV cells is the high cost of the single-crystalsubstrate that GaAs is grown on. Therefore it is most often used in concentratorsystems where only a small area of GaAs cells is needed.

2 . Thin Film Materials

In a thin-film PV cell, a thin semiconductor layer of PV materials is deposited on low-costsupporting layer such as glass, metal or plastic foil. Since thin-film materials have higherlight absorptivity than crystalline materials, the deposited layer of PV materials is extremelythin, from a few micrometers to even less than a micrometer (a single amorphous cell canbe as thin as 0.3 micrometers). Thinner layers of material yield significant cost saving.Also, the deposition techniques in which PV materials are sprayed directly onto glass ormetal substrate are cheaper. So the manufacturing process is faster, using up less energyand mass production is made easier than the ingot-growth approach of crystalline silicon.However, thin film PV cells suffer from poor cell conversion efficiency due to non-singlecrystalstructure, requiring larger array areas and increasing area-related costs such asmountings.Constituting about 4% of total PV module shipments of US4, the PV industry sees greatpotentials of thin-film technology to achieve low-cost PV electricity.Materials used for thin film PV modules are as follows:the material, how much of the sunlight can be successfully converted into electricity is measured by the   concept ofenergy conversion efficiency.

2 .1 Amorphous Silicon (a-Si)

Used mostly in consumer electronic products which require lower power outputand cost of production, amorphous silicon has been the dominant thin-film PVmaterial since it was first discovered in 1974.Amorphous silicon is a non-crystalline form of silicon i.e. its silicon atoms aredisordered in structure. A significant advantage of a-Si is its high lightabsorptivity, about 40 times higher than that of single-crystal silicon. Thereforeonly a thin layer of a-Si is sufficient for making PV cells (about 1 micrometer thickas compared to 200 or more micrometers thick for crystalline silicon cells). Also, a-Si can be deposited on various low-cost substrates, including steel, glass andplastic, and the manufacturing process requires lower temperatures and thus lessenergy. So the total material costs and manufacturing costs are lower per unit areaas compared to those of crystalline silicon cells.Despite the promising economic advantages, a-Si still has two major roadblocks toovercome. One is the low cell energy conversion efficiency, ranging between 5-9%, and the other is the outdoor reliability problem in which the efficiency degradeswithin a few months of exposure to sunlight, losing about 10 to 15%.The average price for a a-Si module cost about $7 per watt in 1995.5

2 .2 Cadmium Telluride (CdTe)

As a polycrystalline semiconductor compound made of cadmium and tellurium,CdTe has a high light absorptivity level -- only about a micrometer thick can absorb90% of the solar spectrum. Another advantage is that it is relatively easy and cheapto manufacture by processes such as high-rate evaporation, spraying or screenprinting. The conversion efficiency for a CdTe commercial module is about 7%,similar to that of a-Si.The instability of cell and module performance is one of the major drawbacks ofusing CdTe for PV cells. Another disadvantage is that cadmium is a toxicsubstance. Although very little cadmium is used in CdTe modules, extraprecautions have to be taken in manufacturing process.

2.3 Copper Indium Diselenide (CuInSe2, or CIS)

A polycrystalline semiconductor compound of copper, indium and selnium, CIShas been one of the major research areas in the thin film industry. The reason for itto receive so much attention is that CIS has the highest “research” energyconversion efficiency of 17.7% in 1996 is not only the best among all the existingthin film materials, but also came close to the 18% research efficiency of thepolycrystalline silicon PV cells. (A prototype CIS power module has a conversionefficiency of 10%.) Being able to deliver such high energy conversion efficiencywithout suffering from the outdoor degradation problem, CIS has demonstrated thatthin film PV cells are a viable and competitive choice for the solar industry in thefuture.CIS is also one of the most light-absorbent semiconductors of 0.5 micrometers canabsorb 90% of the solar spectrum.CIS is an efficient but complex material. Its complexity makes it difficult tomanufacture. Also, safety issues might be another concern in the manufacturingprocess as it involves hydrogen selenide, an extremely toxic gas. So far, CIS is notFundamentals of Photovoltaic Materialscommercially available yet although Siemens Solar has plans to commercialize CISthin-film PV modules.

Heat Transfer of Solar PV Panels

Because solar PV panels interact with their environment and their ref is so low, they passively absorb about 80% of the incoming solar irradiance as heat. This would not besuch a problem if not for a 0.5% efficiency loss of the solar PV panels associated with a1◦K increase of the cell temperature. Because the highest temperatures of solar PVpanels recorded are about 70 ◦C, this efficiency loss can be very noticeable, especiallytrue for yesterday’s PV arrays that have such a low efficiency to begin with. Therefore,heat transfer plays an important role in the actual output of PV arrays.The three modes of heat transfer are involved with the solar PV array. The mainenergy input is solar irradiance in the form of shortwave radiation. The solar panel undergoes heat removal by convection, radiation, and conduction. However, the heat conductedis negligible because of the small contact area between the solar array and the its structural framework. The heat removed from the panel is in the form of longwave radiation due to the much colder temperature of the panel compared to the Sun. A schematicof this heat transfer mechanism is shown. It is worth noting that some solararrays have an anti-reflection coating to decrease reflection losses and increase actual solarirradiance incident on the panel

The temperature of each individual PV cell is a function of its materials, configuration,time of day, rotation of the Earth and environmental factors such as wind, temperature, cloud cover and humidity. To determine the temperature of the solar PV panel acomprehensive heat transfer analysis must be performed.

Cooling Ducts

Brinkworth and Sandberg [2006] calculated the optimal length (L), the hydraulic diameter(Dh), and the width (H) of a cooling duct attached to the back of a solar PV panelwhich would reduce the most heat. They accomplished this by simultaneously solving the external and internal heat transfer equations. The external equationrepresents the heating of the surroundings from the front of the panel while the internalequation represents the heat transferred into the cooling duct. Their model was validated with measurements from a full-scale rig. Factors influencethe temperature of the PV side of the duct are include radiation losses, and decreasingcoefficients of heat transfer until the flow becomes fully developed.